When a plurality of power supply units such as inverters are connected to each other in parallel, and electric power is supplied from the power supply units to a common load, it is necessary that a balance of electric power that is output from the power supply units be secured.
Conventionally, in the electric power balance of the power supply units, it is common that droop control is used that controls an output frequency “co” based on active power P and output voltage “V” based on reactive power “Q”, on the basis of information on the active power “P” and the reactive power “Q” in the output of the power supply unit.
In the droop control, communication between the power supply units is not needed, and there is an advantage that each of the power supply units can adjust the output electric power autonomously, however, it is difficult to secure the desirable electric power balance due to of the effect of power line impedance.
On the other hand, a method is known by which an electric power balance is improved by transmitting and receiving average values of active power and reactive power that are output from the power supply units between the power supply units by wired communication each other.
In addition, a method in known in which communication between the power supply units is performed by a mesh network using radio. In this method, each of the power supply units grasps communication paths between all nodes that constitute the mesh network, and communication using the shortest path is always performed by updating path information periodically. As a result, communication having high reliability can be performed.
In the above-described method using the wired communication, there is a problem that cost for laying a communication line between the power supply units that are installed at a distance from each other increases. In addition, in the method using the radio communication by the mesh network, there is a problem that the processing becomes complicated.